Short Notes on Synapse

Synapse

Synapse is the junction between two neurons. It is a functional connection between nerve cells and some other cells.

Synapse is the junction between two neurons. It is a physiological continuity between two nerve cells. Thus synapse is a part of the nervous system. It serves as structural basis of communication between the neurons in the CNS and between muscle cells and neurons in the PNS. Based on the mode of impulse transmission, the synapse is classified into two:

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  • Chemical synapse 
  • Electrical synapse 

Here, let’s learn more about the meaning and function of synapses. 

Table of Contents

What is Synapse? – Synapse Meaning

Synapse Definition – Synapse is a junction between two neurons or a neuron and a target or effector cell such as a muscle cell. It permits the transmission of electrical or chemical signals.

The synapse is formed between presynaptic and postsynaptic neurons. It is known as the neuromuscular junction between a neuron and muscle.

The conduction of nerve impulses from an axon terminal of a neuron to dendrites of the next neuron occurs through a synapse. It can be electrical or chemical.

Synapse

The synaptic cleft is the name for the tiny space that exists between the membrane of the postsynaptic cell and the axon terminal of the presynaptic neuron. Presynaptic terminals are specialised in mediating the rapid fusion of synaptic vesicles following calcium influx. Specialized receptors are also present in the postsynaptic terminal. In a matter of microseconds, the neurotransmitter diffuses across the synaptic cleft and binds to particular receptors.

Synapse and Neurotransmitters

Neurotransmitters are chemicals that are released from neurons after an action potential at the nerve endings. They travel across synapse, exciting or inhibiting the target neuron. It aids in transferring messages in the form of nerve impulses from the presynaptic membrane to the postsynaptic membrane in the neuron. Various types of neurons make use of different transmitters and hence impart different effects on its targets.

Neurotransmitters are of two types –

  • Inhibitory neurotransmitters – Example: GABA, Glycine
  • excitatory neurotransmitters – Example: Glutamate, Acetylcholine, Aspartate

Structure of Synapse

A typical synaptic structure has a presynaptic neuron, postsynaptic neuron and a synaptic cleft.
The presynaptic neuron functions in storing and releasing neurotransmitters while the postsynaptic neuron participates in the receiving neurotransmitters. Each branch of the axon terminal has several vesicles like the granular, agranular vesicles, cisternae, coated vesicles and endosomes.

The agranular vesicles are called synaptic vesicles. They have the neurotransmitter and are spherical, 35-50 nm in diametre. The synaptic vesicles have small molecule neurotransmitters such as glycine, glutamate etc.

The granular vesicles have a diametre of about 80 nm to 160 nm and usually have neuropeptides. Actin filaments are essential for movement of synaptic vesicles towards the presynaptic membrane to fuse. A significant phosphoprotein synapsin I regulates the neurotransmitter release in the synaptic cleft.

The presence of fodrin and microtubules in addition to F-actin and synapsin are significant in presynaptic terminal. The area in presynaptic ends that are formed by the membrane and a dense collection of proteins that communicate neurotransmitter release is called active zone. It is here where the initial events of synaptic transmission occurs. The postsynaptic section has a higher concentration of receptor proteins or ligand ion channels to receive neurotransmitters.

The association of neurotransmitters to the receptors induce the flow of ions to the post synaptic neurons that eventually leads to the formation of IPSP and EPSP. In the case of glutamate (excitatory synapse neurotransmitters), the acetylcholine associates with its corresponding receptors to cause the flow of positive ions towards the post synaptic cell wherein the association of inhibitory neurotransmitter with its receptor causes the opening of ion channels.

Classification of Synapse

Based on anatomy, synapses can be classified into three types:

  1. Axoaxonic synapses are those where the axons of two different neurons meet. 
  2. An axodendritic synapse is where the axon of one neuron terminates on the dendrite of another neuron. 
  3. Axosomatic synapse is where the axon of one neuron attaches to the soma (cell body) of another neuron. 

Based on function or physiology, synapses are of two types – electrical synapses and chemical synapses. 

Also Check: Difference between Electrical and Chemical Synapses

Chemical Synapse and Its Functions

Signal transmission at synapse is a chemical process. Here, let us see how the pre and post synaptic neurons aid in nerve impulse transmission. 

  • Chemical synapses are more common. The transmission of nerve impulses through chemical synapses is mediated by neurotransmitters.
  • There is a fluid-filled space between the two neurons called the synaptic cleft. The nerve impulse cannot jump from one neuron to another.
  • Axon terminals have a knob-like structure, which contain synaptic vesicles.
  • When the action potential reaches the terminals, the synaptic vesicles from the terminal of the presynaptic neuron release neurotransmitters at the synaptic cleft.
  • Neurotransmitters bind to the receptors present in the postsynaptic membrane.
  • This results in the opening of voltage-gated channels and the flow of ions. This causes a change in the polarisation of the postsynaptic membrane and the electric signal is conducted across the synapse.
  • Neurotransmitters can be inhibitory or excitatory. One neurotransmitter can initiate different responses in different cells.
  • If there is a net flow of positively charged ions inside the cell, then the neurotransmitter is excitatory and it results in generating the action potential. This is called EPSP or excitatory postsynaptic potential.
  • When the membrane potential becomes more negative, the membrane becomes hyperpolarized and the action of neurotransmitter is inhibitory. They generate IPSP or inhibitory postsynaptic potential.
  • Once the neurotransmitters get attached to the receptor they are either acted on by enzymes or taken back and recycled to terminate the signal after it is conducted forward.

Electrical Synapse and Its Functions

  • Electrical synapses are faster than chemical synapses.
  • The presynaptic and postsynaptic neurons are in close proximity and they form gap junctions. There is a physical connection between pre and postsynaptic neurons at the gap junction by protein channels.
  • These gap junctions allow the direct flow of ions and the transmission of an electric signal across the electrical synapse is similar to the conduction of impulse in an axon.
  • Electrical synapses are not as flexible as chemical synapses as they cannot turn the excitatory signal to the inhibitory signal.
  • It is found in some lower invertebrates. In humans, it is found between glial cells.

The synapse, in this way, ensures the conduction of nerve impulses in the correct direction and prevents random stimulation.

Properties of Synapse

The following are the 5 basic properties of a synapse:

  1. One-way conductions – At synapse, impulses can only travel in one direction which is from the presynaptic neuron to the postsynaptic neuron.
  2. Synaptic delay – A brief delay in transmitting impulses through the synapses is called synaptic delay. This is due to the time taken for the release and action of neurotransmitters.
  3. Fatigue – Synapses become the site of fatigue during prolonged muscular activity, along with Betz cells found in the motor area of the frontal lobe of the cerebral cortex. The depletion of acetylcholine is what causes fatigue at synapses.
  4. Electrical property – Electrical properties of the synapse are the IPSP (inhibitory postsynaptic potential) and EPSP (excitatory postsynaptic potentials.
  5. Summation – When multiple presynaptic excitatory terminals are stimulated simultaneously or when a single presynaptic terminal is repeatedly stimulated, the effects are combined or the EPSP gradually increases.

Synaptic transmission

Synaptic transmission is a biological process by which a neuron communicates with a target cell across a synapse. A neurotransmitter is released from the pre-synaptic neuron during chemical synaptic transmission, and the neurotransmitter then binds to particular post-synaptic receptors. Electrical synapses allow direct, passive current flow from one neuron to the next. 

Electrical synapses are special ion channels connecting the pre and postsynaptic cells which provide a low-resistance route for the flow of electrical current between two cells. On the other hand, in the chemical synapse, an action potential in the presynaptic neuron results in the release of neurotransmitters which trigger the current flow in the postsynaptic cell.

Differences between Electrical and Chemical synapse

Electrical synapse Chemical synapse 
Transmitting agent
Ion current Chemical transmitter
Distance between pre-synaptic and post-synaptic cell membranes
3.5 nm 20-40 nm
Is there cytoplasmic continuity between pre- and postsynaptic cells?
Yes No
Transmitting direction
Bidirectional usually Unidirectional
Components
Gap-junction channels Postsynaptic receptors, Presynaptic vesicles and active zones

 

This was a brief note on Neurons and Synapse. Explore notes on neural communication and other important concepts related to NEET, only at BYJU’S.

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Frequently Asked Questions

Q1

What is meant by synapse?

A synapse is a junction between two neurons. It is at these junctions that the axons or some other parts of a neuron ends on the dendrites, axon or soma of another neuron or a gland cell in some other cases.

Q2

How are neurons different from synapses?

Neurons transfer signals to dedicated target cells whereas synapses are the mediate this process.
Q3

What is the function of synapses?

The main function of the synapse is to transmit the impulses, i.e. action potential from one neuron to another. However, some of the synapses inhibit these impulses. Thus based on functions, synapse can be divided into two – excitatory synapse (excitation function) and inhibitory synapse (inhibition function).
Q4

What is the synaptic cleft?

A synaptic cleft is defined as a tiny fluid-filled gap between two nerve cells. It is where the neurotransmitter acetylcholine diffuses.
Q5

What causes synapses in the brain?

When an electrical impulse passes through the axon, it creates a synapse by releasing neurotransmitters.
Q6

What is released from synapses?

Neurotransmitters are released in response to neural activity from the presynaptic neurons that bind with specific receptors to bring about changes in postsynaptic neurons.
Q7

What is synaptic end bulb?

The axon terminal tip enlarges and acquires the name synaptic end bulb near the end of the terminal, which is closest to the muscle fibre. The motor neuron’s synaptic end bulb makes up the nervous system portion of the neuromuscular junction.

Comments

  1. Explain synapse transformation.

    • Neurons or nerve cells communicate with one another at the synapse; it is the point of communication between two nerve cells or between nerve cells and a muscle or gland. A synapse comprises presynaptic and postsynaptic terminal. The presynaptic terminal is where the electrical signal is converted into chemical signal. The postsynaptic terminal comprises special receptors. The postsynaptic neuron is involved in integrating the signals it receives to determine the subsequent set of actions.